How Do Car Brakes Work?

Your brakes are having problems and you want to repair them? Before you can service your car, you'll need to know how the parts of your car's brake system.

How Do Car Brakes Work?
The braking system on a vehicle operates under hydraulic pressure. It starts with a master cylinder located in the engine compartment. Filled with a specialty hydraulic oil (brake fluid), when the brake pedal is manually depressed, it sends a plunger into the master cylinder that builds up hydraulic pressure. The pressure is then transferred through a series of steel brake line plumbing fixtures, to each individual wheel, with another hydraulic component that is activated by the hydraulic pressure. Balancing the correct amount of hydraulic pressure to the front and rear axle, and then the left and right side, are a combination valve and proportioning valve integrated into the steel brake line plumbing. In order to achieve optimum performance from the master cylinder and steel brake lines, the master cylinder must be full of the brake fluid/hydraulic oil, and the steel lines need to be full. Once the plunger is forced into the master cylinder (when stepping on the brake pedal), the least amount of applied pressure is used to activate the other hydraulic components. Cracks, breaks, air pockets, or leaks in the steel brake line plumbing will result in loss of hydraulic power since pressurized brake fluid will purge out of the breach, causing eventual brake failure.

A power booster is located up behind the master cylinder that sends the plunger into the master cylinder mechanically. This is what is referred to as power brakes. Releasing the brake pedal relieves the hydraulic pressure, which allows the integrated pistons to return to their idle position, allowing the vehicle to move freely again.

Friction is another science applied to the braking system of a vehicle. A vehicle starts with hydraulic pressure that is transferred to the friction components. The friction components are what stops the vehicle, but without hydraulic pressure, they are useless. Another application of friction used in the braking systems is the rubber of the tires applied to the roads when stopping the vehicle.
Disc Brakes
Disc brakes are commonly found on the front wheels of all vehicles. More and more, disc brakes are being integrated into the rear wheels on many vehicles manufactured today, replacing drum brakes.

Disc brakes use a disk called a rotor, which is positioned vertically in each wheel. On the outside and inside edge of the rotor, in a concentrated location, is a caliper. The caliper is the hydraulic component of the disc braking system. A caliper piston or pistons extend out of the caliper piston bore under applied hydraulic pressure and contact the friction components (in this case, brake pads). Brake pads are positioned inside the caliper and on each side of the vertical rotor. As the piston extends, it literally squeezes the brake pads against the outer and inner surface of the rotor, slowing down the car. An easier way to think about disc brakes is catching a frisbee. The frisbee is the rotor. The fingers on your hand are the brake pads. The hydraulic pressure of the caliper piston is the motion of your fingers closing against the frisbee to catch it, creating friction.

Drum Brakes
Drum brakes are simpler than they appear, but they're more involved to work on and there are a lot more parts involved. Most commonly found in the rear, drum brakes are being replaced by disc brakes in many automobile manufacturing companies. In older vintage cars of the 1950s and '60s, models with manual brakes (no power booster assist) used four-wheel drum brakes. The braking response was decent for its day, but it did not come close to today's standards.

Drum brakes use a small hydraulic component called a wheel cylinder that is activated by the hydraulic pressure. The cylinder sits sideways between two brake shoes. When the hydraulic pressure of stepping on the brake is applied, small bores on the left and right side of the wheel cylinder protrude outward. The bores contact the friction material, in this case brake shoes, which then get pushed outward and contact the inside surface of encasing steel called a drum.

How Brakes Work